When I heard that Porter Airlines was temporarily offering $20 discounts off their Boston-to-Toronto tickets, I jumped at the chance to check out the Canadian city. After all, the tickets already seemed like a bargain: They regularly sell for $99 to go from Logan Airport to Toronto City Centre Airport.

Of course, I figured the bill wouldn’t end there, as the airline would need to add various taxes and fees, as usual. But I still suffered from a bit of sticker shock when I saw the total cost of two round-trip tickets.

With the discount, they had a face value of just under $320. With the fees, the final price shot up to $510. That’s an increase of nearly 60 percent. The return-trip fees of $150 practically matched the price of the return-trip tickets, dollar-for-dollar.

Although I knew this wasn’t Porter’s fault, I still called the Canadian airline for an explanation. Spokesman Brad Cicero patiently walked me through the taxes and fees, explaining the origins for the bulk of the nearly $200 in extra charges.

Many of the Canadian charges only get attached on departing flights. There are the NAV fees, which totaled $50 for the two round-trip tickets. That’s a Canadian charge for air-traffic control costs there. Cicero says our country’s comparable tax is the transportation tax, which cost us $27.50 for the trip.

The airport charged its own “improvement fee” of $39 for two tickets. Apparently, that’s cheaper than the fee over at the much larger Pearson International Airport on Toronto’s outskirts, but considerably less than the $9 passenger facility charge that we paid for two departures out of Logan.

Then there’s Canada’s Air Travellers Security Charge, affectionately known as the ATSC, which totaled $24.80 for the two tickets. Cicero says this covers airport screening and related efforts. Again, the similar U.S. security tax was much lower: $5 for the two tickets.

The bottom line: Most airplane ticket fees are not directly correlated to the actual price of the ticket. So a big portion of your total bill will most likely end up going to taxes when you buy inexpensive plane tickets like the ones that Porter sells for the Boston-Toronto route.